The surface treatment of plate-shaped workpieces, particularly printed circuit boards, can provide problems when the latter contain holes which are extremely fine. In particular holes in which the ratio of hole diameter to hole length is extremely small, for example 1:10 or less, treatment with the treatment agents can only be carried out with difficulty as these can only be brought into the holes by means of special measures. Such holes are preferably located in multi-layer printed circuit boards, in which a plurality of electrical circuit layers are stacked one above the other and glued together. In this case the holes serve to provide a plurality of conductive connections between the inner and outer layers and if necessary also between the inner layers themselves. In most cases the ends of the printed circuit lines are for this purpose in the form of circular or rectangular plating surfaces. The holes pass through these plating surfaces. For purposes of electrical contacting of the individual circuits with one another, the walls of the holes are metallised. The copper sleeve bearing on the bore wall surfaces thus produced represents the electrical contacting between two or more associated contacting surfaces of the electrical circuits.
Various treatment steps are necessary for metallisation, such for example as cleaning, conditioning, activation, metallising and drying. When the holes are bored, for example, plastic emerges at the resin areas, each only a few micrometers thick, between the metallic surfaces of the electrical circuits in the interior of the printed circuit board; the plastic deposits as a smeared resin film on the bore surfaces of the metal layers to be contacted. In the case of the more usually used FR4 printed circuit boards (laminates with flame-retardant resin and glass fibre mats as reinforcement) the plastic consists of epoxy resins. When such smearing covers roughly 50% of the wall surface of the hole, the conductive connection at the contacting surfaces is already at risk. If a degree of smearing, even in one single plating hole, reaches about 80%, then the entire board should be rejected. When there are a plurality of plating holes for a larger multi-layer printed circuit board and in view of the extremely high manufacturing costs it is therefore clear that this smearing can lead to considerable costs due to waste, while in addition recognition of the smears after drilling is only possible with considerable outlay, and then only in individual holes.
Difficulties in the removal of resin smearing increase as an increasing proportion of small and extremely small holes (for example smaller than 0.4 mm diameter) have to be processed. For example, printed circuit boards are already being produced with a thickness of 7.3 mm and with holes of a diameter of 0.35 mm (ratio of the bore hole diameter to the length roughly 1:21).
Experience has shown that the flow speeds of treatment agent achievable in through plating holes with diameters with less than 0.3 mm and with lengths above 4.5 mm is no longer sufficiently high, so that sufficient exchange of materials at the wall surfaces of the holes is no longer guaranteed. In this case the holes behave almost like capillaries, which in the most favourable case are still wetted by the treatment agents, yet through-flow is only achievable with difficulty. Particularly when the peripheral surfaces of the holes are rough, wetting and through-flow are practically impossible. This is above all the case when operation is with liquid treatment agents, which have a higher viscosity than for example water.
In order to enable wetting and through-flow of such fine holes, it is therefore proposed among other things to pass the workpieces through the treatment plant in a horizontal operational position. In order in this case to bring about forced flooding of the holes, a flow of the liquid treatment agent (splash, spray jet) is directed from a nozzle system against the flat surfaces of the workpieces. During treatment, the workpieces are arranged horizontally and passed in a horizontal transport direction through the plant. Transport from one treatment station to the next is effected via rollers or by travelling carrier clamps, the latter grasping the individual workpieces at the edge or at the edge of two opposite sides in a pincer fashion.
There is for example disclosed in the publication DE 26 06 984 B2 a method for chemical cleaning of plating holes in printed circuit boards with the help of an aggressive acid as a cleaning medium, dissolving epoxy-resin smearing on the hole wall, in which the horizontally guided printed circuit board, after the plating holes have been bored, passes at a constant speed in a closed active chamber over a splash zone, which is formed by a slotted tube located beneath the transport path and perpendicularly to the transport direction, and out of which chemical cleaning agent emerges under pressure, so that the plating holes on the printed circuit board are thereby intensively rinsed. Thereafter the printed circuit board is blown down by a sharp air jet and rinsed.
There is also disclosed in the German Patent DE 30 11 061 C2 a method for intensifying rinsing and cleaning processes for moulded parts provided with perforations, such as printed circuit boards and etched moulded parts, in automatic rinsing and cleaning machines provided with nozzles, in which the moulded parts, treated in active chambers on a transport path, are passed through rinse chambers and are treated from below with rinsing agent, and in which rinsing agent splash zones are additionally used in the rinsing chambers and, in order to generate the rinsing agent splash zones, at least one slotted tube located underneath the transport path is used, from which the rinsing agent emerges under pressure.
In the German Disclosure Document DE 35 28 575 A1 there is described a method of cleaning, activating and/or metallising bore holes in horizontally guided printed circuit boards, in which the printed circuit boards pass at a constant speed over a splash zone, which is formed by a nozzle located beneath the transport path and perpendicular to the transport direction, from which the liquid treatment agent is passed in the form of a standing wave on to the underside of the printed circuit board.
According to the present publications in prior art there is understood by the term "splash" a continuously flowing stream (jet) of the various treatment agents, particularly the liquid but also the gaseous agents, the flow emerging at the highest possible speed from splash nozzles, and impinging on the plate-shaped workpiece (printed circuit board) to be treated. In addition to these methods, methods are also known in which the liquid treatment agents are sprayed or squirted by means of spraying or squirting devices against the workpieces to be treated. During squirting or spraying the treatment agents are intensely permeated by air.
By means of this splashing, spraying or squirting of the liquid treatment agents against the workpiece, the holes located in the workpiece are forcibly flooded, i.e. maximum possible relative movement is produced between the wall surfaces of the holes and the treatment agents flowing through the holes. This relative movement is known to be the most important condition for an effective exchange of materials at the phase limit between solid body (wall surface of the hole) and treatment agent.
The known methods using the splash technique are therefore not suitable for flooding through extremely fine holes. In explanation, a concrete example from operational practice will be described:
In a printed circuit board with an average 64 holes per cm.sup.2 surface with a hole diameter of 0.3 mm, the sum of the cross-sectional areas of the holes comes to about 4.5 mm.sup.2 per cm.sup.2 of printed circuit board surface. The ratio of the average open passage area in the printed circuit board to the closed proportion of the plate surface thus comes to about 4.5%.
The splash flow impinges at high speed perpendicularly on to the flat horizontal surface of the printed circuit board. As the open surface of the printed circuit board in the region of the holes comes to only a small proportion of the overall area, the impinging flow splits into two almost equally large flow components moving apart from one another in opposite directions, and located parallel to the transport direction. These flows run upon and along the plate surface. The small proportion of the cross-sectional openings in the workpiece pierced by holes gives a sufficient indication of the small proportion of the splash jet which flows through the holes, despite forced flooding.
Furthermore, the treatment agent flowing parallel to the surface prevents the formation of flows through the holes, as the penetrating flows are directed perpendicularly to the liquid flowing along the plate surface. The splash jet, contrary to its intended effect, thus exerts an inhibiting effect on the establishment of the flooding of the holes.
In addition the arrangement described in the German Disclosure Document DE 39 16 694 A1 for treatment and/or cleaning of material, particularly printed circuit boards provided with holes, with the aid of a liquid applied in a splash configuration, whereby there is provided on one side of the material to be cleaned or treated a splash zone and on the other side of the material a suction zone, does not lead to the desired successful conclusion. In this case the forced flooding is in fact improved by means for removal by suction of the liquid treatment agent forced through the holes. Yet by means of this arrangement only the resistance which is opposed to the traversing treatment agent by possible rough bore wall surfaces can be overcome.
The method described in the German Patent DE 40 17 380 C1 for rinsing holes in articles to be electroplated, particularly plate-shaped and multi-layer articles such as printed circuit boards, does not remove the problems in through-flow of extremely fine holes. In this method the cleaning or rinsing liquid is passed under pressure through the holes; the speed of the liquid flow passing through the holes is alternatingly increased or reduced.
The splash nozzles described in the publications cited above consist of an elongate rectilinear casing, which is arranged perpendicularly to the transport plane of the printed circuit boards passing through horizontally, as well as parallel thereto, and has a longitudinal slot, from which the emerging splash jet impinges perpendicularly on the flat surface of the plate-shaped workpieces. It can be seen from the above that during the overall treatment time of the workpiece only a fraction of the plate surface and thus only a fraction of the holes located therein are intensely irradiated with the treatment agent. Treatment of the workpiece is effected at irregular points and intervals on the surface, so that, due to the horizontal transport of the workpiece through the plant, treatment of all surface areas is only possible in a gradual manner. This however contains the fact that during a considerable portion of the overall treatment time of the workpiece in the treatment plant, no or practically no flow of treatment agent is generated in the holes.
The German Patent DE 39 05 100 C2 describes a method of chemical or electrolytic surface treatment of plate-shaped workpieces provided with small openings, particularly of large-area printed circuit boards provided with bore holes, for printed circuits, in aqueous solutions, the workpiece executing two separate vibratory movements independent of one another, the first vibratory movement extending at right angles to the surface of the workpiece and the second vibratory movement being effected in roughly the same direction as the first vibratory movement and simultaneously therewith, the frequency of the first vibratory movement being considerably lower than that of the second vibratory movement, which represents an intensely pulsating rapid-sequence vibrational oscillation, the amplitude of the first vibratory movement being considerably greater than that of the second vibratory movement.
This method is suitable only for treating plate-shaped workpieces in installations with baths in which the workpieces are submerged vertically and raised out of said baths again after the treatment, whereupon the workpieces are next transported to the next treatment point. A particular disadvantage in this method is that the vibrational oscillations which are intended to favour the through-flow of the holes of the workpieces can scarcely be transmitted to all the workpieces which are submerged in the bath. When the conventional dip baths are used which in part reach a depth of 1.80 m, the workpieces secured at the lower end of the workpiece holder could only be set in vibration with sufficient amplitude by means of an unreasonably high outlay. In addition, bores in the workpieces can scarcely be flooded, as no flood tubes are provided. Due to the movement of the workpieces, any flood tubes present would have to maintain a relatively large spacing from the workpieces. The workpieces could only with difficulty be introduced from above into the bath container, if flood tubes were located at a small spacing from the workpieces. Therefore the problem underlying the present invention is to avoid the disadvantages known in prior art and to find a method of treating plate-shaped workpieces provided with extremely small holes, particularly of printed circuit board, by means of a liquid or gaseous treatment agent.
By means of the measures according to the invention, the wall surfaces of holes in plate-shaped workpieces, particularly of printed circuit boards are treated by means of liquid or gaseous agents in such a way that in terms of fluid mechanics the highest possible relative movement is produced between the wall surfaces of the holes and the liquid or gaseous treatment agents flowing along these.